Methemoglobinemia is a disease that, encompases many different things. It is similar to diabetes, which is one name for a great number of causes, symptoms, etc. It is just that one of the causes of Methemoglobinemia is a recessive gene, and another is a dominant gene. Dominant and recessive are terms only used when talking about the chance of inheriting or expressing a trait.

In one branch of my family, there is a gene that is dominant for expressing six fingers.
In the amish community, there is a gene that is recessive for expressing six fingers.

You can have either gene and express the same condition. However, the recessive trait tends to be expressed less often than the dominant one.